The invention relates to a dispersing device consisting of a container for accommodating and processing a substance to be dispersed, a flow generator driven by a first shaft, a grinding device containing a grinding medium and having a casing with openings, through which the substance to be dispersed can pass due to the flow generated by the flow generator, and an agitator mounted inside the housing which can rotate relative to it. The invention also relates to a process for the pre-dispersion and fine dispersion of a fine to very fine, solid particulate material in a dispersant in a container for accommodating the substance to be dispersed, where pre-dispersion is performed using a dissolver and fine dispersion using an agitating ball mill.
A device of this kind disperses fine to very fine, solid particulate constituents in the liquid phase.
Three sub-steps occur simultaneously during the dispersion process:
1. Wetting of the surface of the solid material to be incorporated by the liquid constituents of the mill base,
2. Mechanical separation of agglomerates into smaller agglomerates and primary particles, and
3. Stabilization of primary particles, agglomerates and aggregates to prevent renewed clumping (flocculation).
Although the following description primarily relates to the dispersion of paints and coatings, this processing technique can also be applied in a similar manner in other fields (e.g. biology, food processing technology, pharmacy, agrochemistry, ceramics industry and the like).
When dispersing paints, for example, it is of economic interest to minimize the use of relatively expensive primary colorant particles. The better the dispersion, the more intense are the colour effect and gloss. Thus, good dispersion can reduce, for example, the use of expensive primary colorant particles by using cheaper secondary particles. In the ideal situation, each primary particle is wetted separately.
A dispersing device which exhibits the characteristics of this generic description is known from EP 526 699 A1.
A grinding device of this kind is also known from U.S. Pat. No. 5,184,783. This patent specification presents an agitating submersible mill which disperses according to the circulation process. It essentially consists of a wear-resistant strainer filled with grinding balls which is submerged in a double-walled container. A cylindrical drive shaft runs through the centre of the strainer. This drive shaft drives the bar-shaped agitator mounted inside the strainer. The walls of the strainer exhibit sieve-like perforations.
In order to enable the circulation of the grinding medium through the strainer, the drive shaft drives a flow generator in addition to the agitator. This flow generator must be positioned outside the strainer in order to ensure adequate flow. Thus, the drive shaft penetrates the strainer. A separating and sealing system is fitted at the point of penetration to prevent the grinding media from escaping from the strainer.
The central position of the flow generator has definite advantages in terms of fluid mechanics, because it ensures uniform circulation throughout the container.
However, in order to carry out an economical dispersion process using the dispersing device known from the prior art, the substance to be dispersed must be pre-dispersed. Pre-dispersion is preferably performed using a dissolver disk due to the fact that optimum pre-dispersion is indispensable from an economic standpoint, particularly in the case of agglomerates which are difficult to disperse and require the use of the grinding device during subsequent processing. An inadequately pre-dispersed product not only necessitates longer running times of the grinding unit known from the prior art, but it also frequently happens that the desired fineness is not attained. As a rule, faults or errors in pre-dispersion cannot be compensated for by other systems, particularly because inadequately pre-dispersed products clog the holes in the strainer during subsequent use of the grinding device, thus hindering, or even completely stopping, circulation through the strainer.
However, the dissolver can often achieve only pre-dispersion, i.e. the substance to be dispersed can only be dispersed up to a certain degree of fineness. An agitating ball mill or similar grinding device must be used for further dispersion, or "fine dispersion".
When switching from pre-dispersion to fine dispersion, either the container with the substance to be dispersed, the respective drive unit for the machine, or the attachments must be exchanged, or the substance to be dispersed must be pumped out of the container and into a separate agitating ball mill.
In addition, the entire installation must be cleaned when switching products--a change from red paint to white paint, for example. When cleaning the ball mill, it is very difficult to clean the separating and sealing system. The conversion and cleaning processes thus cause extensive idle times and costs.